Yes, you can change the difficulty mid-game. For the first boss, I ended up changing the difficulty from normal to easy so I could beat him. (I suppose I'm not very good at video games. Thing is, I wasn't having fun any more and have reached the stage in my life where making valiant, video-game related efforts feels absurd and embarrassing and I just wanted to get back to enjoying the game again. A fast-forward of sorts. Worked well.)
Once past the first boss, I put the difficulty back to Normal and left it there through the game's end.